1. Field of the Invention
The present invention generally relates to piston type compressors, more particularly, to piston type compressors having a cam plate.
2. Description of the Related Art
A typical piston type compressor is described below. This type or compressor includes a pair of cylinder blocks secured to each other. A front housing is coupled to the front end of the front cylinder block with a valve plate arranged in between. In the same manner, a rear housing is coupled to the rear end of the rear cylinder block with a valve plate arranged in between. A crank chamber is defined between the cylinder blocks. Further, suction and discharge chambers are defined in each housing. The cylinder blocks also include a plurality of cylinder bores and a suction passage defined therein. A double-headed piston is reciprocally housed in each cylinder bore. The valve plates are provided with a plurality of suction ports each corresponding to one of the cylinder bores. Each suction port is selectively opened and closed with a suction valve flap.
As each piston is reciprocated in the associated cylinder bore, refrigerant gas is drawn from an external refrigerant circuit into the crank chamber. The gas is then supplied to the suction chambers in the front and rear housings by the suction passage. The gas in the suction chambers is drawn into each cylinder bore through the corresponding suction port.
In the above described prior art piston compressor, each suction port is arranged on the corresponding cylinder bore at the same position with respect to the center of the valve plate. Further, all the suction valve flaps are arranged extending in the same direction with respect to the rotating direction of the drive shaft. The recent trend of increasing in the number of cylinder bores has resulted in an increased ratio of the cross-sectional area of cylinder bores to the cross-sectional area of the cylinder block. This results in a reduced cross-sectional area for forming suction passages. It is thus difficult to form the same number of suction passages as the number of cylinder bores. In other words, it is difficult to form suction passages so that each corresponds to one cylinder bore.
In a compressor having the above described construction, the pressure in each suction chamber varies from one location to another. This causes the pressure in a cylinder bore close to a suction passage to be different from the pressure in a cylinder bore far from the suction passage. The variation of suction pressures in the cylinder bores is referred to as suction pressure loss. Suction pressure loss results in unstable compression operation.
When low pressure refrigerant gas is drawn into a cylinder bore, the compression ratio needs to be relatively high for compressing the gas to a predetermined discharge pressure. In the state, the compressor takes more time until the refrigerant gas is discharged from a compression chamber. Consequently, the amount of discharge gas is decreased in accordance with the delay of discharging timing for operating the compressor. In short, the variation of the pressure in the suction chambers results in an increased power loss. Further, the temperature of the discharged gas is increased. Thus, the refrigerant capacity of the external refrigerant circuit is degraded.
Further, if a cylinder bore draws lower pressure refrigerant gas, the cylinder bore has a low suction pressure. The cylinder bore therefore draws less refrigerant gas from the external refrigerant circuit. This lowers the flow rate of the refrigerant gas in the circuit thereby increasing the pressure in the evaporator. The elevated pressure in the evaporator degrades the refrigerant capacity of the circuit.